Abstract: In a paper making process the liquid fraction of wood pulp stock liquor is sampled between the bleaching and caustic extraction stages and after caustic introduction to regulate the amount of caustic introduced into the pulp stock liquor. A particular sampling device with a lower pressure chamber extracts that stock liquor from the periphery of a higher pressure pipe and passes the sample to previously known continuous testing apparatus. A particular double screen type interfacing joins the sampling device and pulp stock to allow liquor extraction without excessive wear or plugging of the interface.